|M42 Angénieux Retrofocus lens|
image by Woradate Pattayawan (Image rights)
Retrofocus refers to a type of lens design, pioneered (and originally trademarked) by Pierre Angénieux in the early 1950s, which permits a short lens optical focal length while providing a greater back focus distance. This is typically required in an SLR camera, to allow space for the swinging reflex mirror; and also in some cine cameras using small image formats, to permit space for the shutter mechanism.
Sometimes known as a "reverse telephoto" design, retrofocus designs typically have outermost lens groups with negative power, with convergent elements behind them. It can be difficult to design a retrofocus lens that is fully free from barrel distortion.
Since the rear element is further away from the film / sensor plane, retrofocus designs also have much lower vignetting (i.e. more even illumination across the image).
- US Patents 2649022, Wide-angle photographic objective lens assembly, filed July 1950 and granted August 1953, and 2696758, Wide-angle photographic objective, filed March 1953 and granted December 1954, both describing a retrofocus lens, at Espacenet, the patent search facility of the European Patent Office. The patents make reference to a French one of February 1950, not in the database at Espacenet.